Arsenic and vanadium compounds have wide-ranging importance in biochemistry, both as poisons and (at least in the case of vanadium) as apparently essential trace elements. Arsenate (V) and vanadate (V) may substitute for phosphate as a substrate in certain phosphorylation reactions, and arsenate has been used extensively to study the sequence of reactions to mitochondrial oxidative phosphorylation, both to gain further insight into the normal mechanisms and to understand the means by which arsenates uncouple the process. Oxidative phosphorylation models usually postulate arsenate esters and/or arsenatophosphates as intermediates, but very little chemical work has been done on these species under conditions approaching a biological environment. Arsenic (III) is implicated in the toxicology of arsenic and is presumed to react with sulfhydryl groups, but no chemical studies of such reactions have been performed. Vanadium (IV) and vanadium (III) have a role in some biological processes, but very little is known of the chemistry of these oxidation states under biological conditions. We propose to continue our study of the rates and mechanisms of hydrolysis reactions involving arsenate (V) and vanadate (V) esters, condensed arsenates and vanadates and arsenatophosphates, and of oxygen exchange reactions involving vanadate (V), vanadate (IV), and arsenate (III) species. We also plan characterization, equilibrium, and rate studies of vanadium (IV) species in neutral solutions and similar studies on arsenic (III) complexes of hydroxyl and sulfhydral groups; finally, we play to study redox reactions involving well-characterized arsenic (V) and arsenic (III) compounds.